1. Field of the Invention
The present invention relates to a positioning apparatus.
2. Description of the Related Art
Along with the recent advance of the information-oriented society, the degrees of integration of, for example, devices and circuits are rapidly increasing. The development of micropatterning technologies has attained such an increase in the degrees of integration. For example, Japanese Patent Laid-Open No. 10-289943 discloses a system which controls a stage using a laser interferometer. This system incorporates an interferometer for measuring the position of the stage, so that one driving axis of the stage corresponds to one measurement light beam axis.
However, to move the stage with a long stroke in this system, it is necessary to attach an interferometer with a large mirror to the stage. This results in a decrease in the dynamic characteristic of a control system of the stage. Assume, for example, that an interferometer for focus direction measurement is set in the exposure apparatus. From the viewpoint of the arrangement relationship with a projection lens, it is geometrically difficult to lay out the light beam axis of one interferometer, such that it allows measurement in the whole movable region of the stage.
To solve this problem, Japanese Patent Laid-Open No. 2000-187338 proposes a technique of using a plurality of interferometers to cover one driving axis of the stage, and to switch them between the off-axis alignment measurement time and the exposure time, thereby reducing the weight of their mirrors. To measure the position of the stage by switching between, for example, two interferometers, a stroke with which both interferometers can simultaneously measure the stage position is determined. Then, the measurement value of one interferometer for measuring the position of the stage in this stroke is preset, as the measurement value of the other interferometer to be selected next.
When a plurality of interferometers are switched during stage driving, a predetermined amount of position error proportional to the moving velocity occurs, during the time interval from reading the measurement value of one measurement interferometer to presetting it as the measurement value of the other interferometer. A variation in this time increases the error to an indefinite value, so the error value at the current position is accumulated. To avoid this problem, Japanese Patent Laid-Open No. 2003-254739 calculates the product of the moving velocity of the stage and the switching execution time, and sets a value obtained by adding the product to the position of the stage, before switching, as the initial value after switching. This makes it possible to reduce errors upon switching at a constant velocity.
Unfortunately, the prior art causes errors due to the switching between, for example, measuring devices, such as interferometers, during stage acceleration/deceleration, because the stage deforms.
In a stage of, for example, a semiconductor exposure apparatus, even a switching error of several nm often becomes problematic. However, it is unavoidable that a stage top plate and a measurement reference mirror, arranged on it, deform during stage acceleration/deceleration. When laser interferometers are switched while the mirror has deformed, the shape of the mirror irradiated with a laser beam changes between the acceleration time, the constant velocity time, and the stop time, resulting in switching errors. Repeating switching during acceleration/deceleration accumulates switching errors.